Growth of the brittle star Ophiura sarsii sarsii in Funka Bay, Hokkaido, Japan

The brittle star Ophiura sarsii with a disc diameter (DD) of less than 9 mm is an important prey item for flathead flounder Hippoglossoides dubius in Funka Bay. To establish an age-determination technique and estimate the growth equation for Ophiura sarsii sarsii (a dominant sub-species of O. sarsii in Funka Bay), we observed ridges on the surface of the arm ossicles. Specimens were collected using a sledge net, their DD measured, and the first arm ossicles examined from scanning electron microscopy images. Because the outermost ridges were formed from November to March, ridges were identified as yearly rings (annuli). The maximum age was 16.83 years (16.46-mm DD), and the largest sized individual was 22.19-mm DD (13.75 years). The best growth equation between DD and age (t) was given by Richard’s model using a minimum Akaike’s information criterion and growth rate comparison: DDt= 24.02 × {1 − 1.70 × exp[− 0.067 × (t + 8.25)]}−1/(−1.70). Small-sized (< 9-mm DD) and large-sized (ca. 15-mm DD) cohorts were composed of multi-year classes, and estimated growth rates from cohort progressions were 2.25 mm/year and 0.98 mm/year, respectively. We presumed that the large-sized cohort could be explained by a high survival rate and slow growth rate after the age of 5 or 6 years.